The invention relates to acidizing an acid-reactive material, such as a carbonate mineral or deposit in or around the borehole of a well, to dissolve or at least chemically alter the acid-reactive material. The invention is particularly useful in matrix and/or fracture acidizations of subterranean carbonate reservoirs, particularly those in which the temperatures are relatively high, e.g., at least about 120.degree. F.
The need for achieving slow acidizations in carbonate reservoirs has been known for more than about 40 years. U.S. Pat. No. 2,059,459 (filed June 5, 1936) mentions that hydrochloric acid tends to be spent before it penetrates any significant distance into the reservoir and its rapid and violent reaction tends to develop insoluble fine solids that impair permeability. The patent suggests injecting both a nonaqueous fluid capable of forming or releasing an acid and a water or brine that ensures that release. U.S. Pat. No. 2,301,875 suggests using an aqueous buffer solution of a weak acid and a weak acid salt which has a relatively high pH and a relatively low rate of reaction due to the low hydrogen ion concentration. U.S. Pat. No. 2,863,832 suggests improving the process of the U.S. Pat. No. 2,059,459 patent by injecting only an oil solution of an organic acid anhydride that forms the acidizing solution in situ without injecting any water. A series of patents by R. E. Dilgren and Dilgren and F. M. Newman, U.S. Pat. Nos. 3,215,199; 3,297,090; and 3,307,630, suggest injecting a hydrolyzable organic halide, such as a halogenated hydrocarbon or ether, mixed with a solvating medium, such as water, to form hydrochloric acid by an in situ solvolysis reaction. U.S. Pat. No. 3,441,085 suggests slowly acidizing a carbonate reservoir by (a) injecting a weak acid or a weak acid solution which is so concentrated that the rate of acidization is impeded by the amount of salts which are precipitated from the concentrated solution, and (b) subsequently injecting water or brine to dissolve the precipitated salts and cause further acid acidization and acid penetration. U.S. Pat. No. 3,630,285 describes a process for slowly acidizing a hot carbonaceous reservoir by injecting water and a water-soluble ester of an organic carboxylic acid (where the reservoir temperature is at least 200.degree. F.) so that a carboxylic acid is formed by the hydrolysis of the ester.
A copending patent application, Ser. No. 813,014, filed July 5, 1977 by E. A. Richardson and C. C. Templeton describes a process for slowly acidizing a reservoir that contains siliceous and/or argillaceous materials. An aqueous solution containing salts of both hydrofluoric and chlorocarboxylic acids is injected so that a mud acid is formed within the reservoir. The chlorocarboxylate ions are hydrolyzed to yield an acid that reacts with the fluoride ions so that a clay-dissolving mud acid is formed. Numerous aspects of the procedures described in application, Ser. No. 813,014, are pertinent to the present invention and those disclosures are incorporated herein by cross-reference.